The invention relates to a layer arrangement for the formation of a coating on a substrate, a coating method for depositing a layer arrangement, as well as a substrate, in particular a tool having a layer arrangement in accordance with the present invention.
The production of efficient tools is mostly realized by the formation of a coating on their surfaces. An important class of such coated tools is in particular, among other things, cutting tools in various embodiments. Typical materials to be coated are, among other things, tool steel and hard metals.
A known problem concerned with the coatings of such materials is both the high oxidation rate under the influence of air, even around 500° C., and the sag at relatively low temperatures (HSS ca. 550° C., hard metal ca. 650° C.).
Thus, preferably ceramic cutting members for the working of steels are used, e.g. ceramic cutting members on the basis of cubic boron nitride. For example for the high speed working of aluminum alloys and grey cast iron, SiN-ceramics are increasingly used. Thereby, ceramic materials prove to be more resistant in comparison to the metallic tool materials. A further improvement in performance can be achieved by a capable coating of the tools.
The hard coatings known from the state of the art are often based on classical compositions such as TiN, TiNC, CrN. These known hard coatings are limited with respect to their field of applications due to their special physical properties, especially with respect to their temperature load capacity. On the one hand, hardness is decreasing at elevated temperatures, and on the other hand, oxidation occurs even at relatively low temperatures, leading to an increasing wear of the coating.
To avoid these problems essentially two classes of coatings have been developed being, both, oxidation resistant in a range up to 1000° C. and having better properties with respect to hardness.
One class of coatings is Al-containing base layers such as AlTiN and AlCrN, wherein, dependent on specific requirements, additional elements can be alloyed. Typical compositions are compositions of the type AlTiXNCO, wherein X is e.g. Cr or another metal.
Another possibility known from the state of the art for improving the performance of coated tools is the combination of classical hard coatings acting as support layers with a finish-layer acting as a top-layer and having a respective function. In particular Si-coatings having a high Si content (10 at % or higher; at % means in the framework of this application “atom percent”) of the type MeSiXNCO (X being an additional metal or B) such as TiSiN have to be mentioned which have an increased temperature load capacity.
In addition, it is for example also known to CVD-methods to deposit on indexable inserts oxidic ceramic-coatings such as Al2O3 using CVD-methods, in order to reduce wearing processes at elevated contact temperatures, in particular while turning on a lathe.
Under investigation are actually also boron-based coatings such as B4C as well as cubic BN coatings. However, cubic BN has the decisive disadvantage that it is extremely difficult to prepare. This is mainly due to difficulties in connection with the growth of the coating as such, as well as due to high internal stress in the coatings.
Regarding high temperature materials, volumetric-ceramic materials on the basis of SiN have been developed in recent years having greater hardness and improved oxidation resistance in comparison with SiC and Si3N4. Their special properties result from complex covalent chemical bonds and are due to low oxygen diffusion rates within the amorphe structure of SiCN.
Up to now it was only partly possible to provide coatings which meet the constantly increasing requirements with respect to mechanical properties, such as hardness and toughness, tribological properties such as affinity to adhesion at elevated temperatures, as well as friction, oxidation resistance and other characteristic properties, especially at extreme temperatures.
Moreover, the above-described improved coatings are procedurally difficult to prepare, so that the coated tools are very expensive and, as a result, from the economical point of view, in many cases coating is not worth while and tools being coated in such a way have only a limited market.